This invention relates to catalysts for removing organic compounds from a gas stream containing the same and processes for removing organic compounds from the gas stream using the catalysts, and particularly to such catalysts as are effective for removing organic compounds diluted in a gas stream and processes for removing such organic compounds from the gas stream using the catalysts.
A wide range of synthetic organic compounds are processed and produced as intermediates and products in chemical plants, with a concomitant leakage or accidental discharge into the environment. Frequently, exhaust gases from waste incinerator plants contain organic compounds. Many such organic compounds are harmful to both the environment and health. Consequently, there is a need to establish an effective emission control technique. Particularly, there is a need to develop effective and practical techniques for controlling emission of volatile organic compounds or halogen-containing organic compounds.
There have been proposed many methods for removing organic compounds from gas streams, including adsorption, direct combustion and catalytic combustion methods. Though the adsorption method may work effectively when it is employed for treating a gas stream having a high content of organic compounds, removal efficiency may deteriorate when it is used for treating a gas stream containing dilute organics. In the case of the direct combustion method, a temperature as high as 800xc2x0 C. or higher is required, which is not economically viable. Furthermore, the direct combustion method suffers from the problem that nitrogen oxides are generated which could result in a source of secondary pollution.
As a catalytic combustion method, for example, Japanese Patent Public Disclosure (KOKAI) No. HEI-4-250825 describes a method for treating a gas stream containing halogen-containing organic compounds, wherein said gas stream is brought into contact with an acidic zeolite. In the method, the halogen-containing organic compounds are treated by contacting the gas stream with an acidic zeolite or an acidic zeolite that is loaded and/or exchanged with at least one metal belonging to the 2nd to 6th periods of the Periodic Table of Elements.
Further, Japanese Patent Public Disclosure (KOKAI) No. HEI-4-284849 discloses a catalyst for decomposing 1,2-dichloroethane and a method for treating a waste gas stream with such a catalyst. In the disclosed method, the 1,2-dichloroethane decomposing catalyst comprises an acidic zeolite or an acidic zeolite which has been loaded and/or ion-exchanged with a transition metal(s).
Further Japanese Patent Public Disclosure (KOKAI) No. HEI-8-38896 discloses a catalyst for decomposing chlorinated volatile organic compounds. According to the disclosure, the catalyst is capable of decomposing chlorinated volatile organic compounds in the presence of steam and oxygen. The catalyst comprises at least one element selected from platinum, palladium and ruthenium as a primary catalytically active substance which is supported on a carrier consisting essentially of zirconia, said carrier supporting further a cocatalyst comprising boron oxide. In the catalyst, the proportion of the primary catalytically active ingredient is, calculated as metal, 0.1 to 5% by weight on the basis of the total weight of catalyst, and the proportion of the cocatalyst is, calculated as B2O3, 2 to 5% by weight on the same basis.
Japanese Patent Public Gazette (KOKOKU) No. HEI-6-87950 describes a process for catalytically reacting a waste gas stream containing hydrocarbons, halogenated hydrocarbons and carbon monoxide, and an apparatus for effecting such a process. The reference particularly describes a process for catalytically contacting a waste gas stream from vinyl chloride synthesis, wherein said gas stream is passed through a first zone including a catalyst for oxidatively decomposing the noxious compounds at 300-800xc2x0 C., and then through a second zone including a catalyst for oxidatively combusting the noxious compounds. The first zone catalyst comprises, as a catalytically active substance, aluminum oxide, silica dioxide and/or a zeolite, which may contain optionally 0.1 to 20% by weight of one or more oxides of elements Ba, Cu, Cr and Ni. The second zone catalyst comprises, as a catalytically active ingredient, platinum and/or palladium, or a combination of platinum and rhodium.
As above-mentioned, prior art references Japanese Patent Public Disclosure (KOKAI) Nos. HEI-4-250825 and 4-284849 disclose methods for treating gas streams contaminated with halogen-containing organic compounds, in which the gas streams are brought into contact with either an acidic zeolite or an acidic zeolite supporting and/or exchanged with at least one metal belonging to the 2nd-6th periods of the Periodic Table of Elements. However, these catalysts are not satisfactory in their gas cleaning performance in that the methods can produce by-products including halogen-containing compounds other than those that are envisaged to be removed through the methods.
On the other hand, the combustion methods disclosed in the above-mentioned references Japanese Patent Public Disclosure (KOKAI) No. HEI-8-38896 and Japanese Patent Public Gazette (KOKOKU) No. HEI-6-87950 require a considerably raised operation temperature exceeding about 500xc2x0 C.
Accordingly, one of the important objects of the invention is to provide improved high-performance combustion catalysts capable of treating a waste gas stream containing organic compounds without yielding undesirable by-products.
A further primary object of the invention is to provide a method in which such a catalyst is employed efficiently for removing harmful or noxious organic compounds present in a gas stream without yielding undesirable by-products. The other objects and advantages of the invention will become clear from the description given below.
The inventors have concentrated their efforts on resolving the above-discussed problems presented by the prior art. As a result, it has been found that a mixture of a zeolite and a metal oxide containing at least one element of the platinum group provides an effective combustion catalyst capable of removing organic compounds in a gas stream without producing by-products.
It has been also found that a similarly effective catalyst is provided by an alumina having such a pore size distribution that, where xe2x80x9caxe2x80x9d represents a pore radius in xc3x85 at the maximum of the pore size distribution curve, the accumulated pore volume of pores having radii in the range of xe2x80x9caxe2x80x9dxc2x125 xc3x85 is at least 65% of the total volume of all the pores, said alumina containing less than 1% by weight of rare earth elements and being loaded with one or more elements of the platinum group. The invention is based on these and incidental findings.
Thus, according to the first aspect of the present invention, there is provided a combustion catalyst for removing organic compounds, which comprises a mixture of a zeolite and a metal oxide containing at least one of the elements of the platinum group.
According to the second aspect of the invention, there is provided a combustion catalyst for removing organic compounds, which comprises an alumina having such a pore size distribution that, where xe2x80x9caxe2x80x9d represents a pore radius in xc3x85 A at the maximum of the pore size distribution curve, the accumulated pore volume of pores having radii in the range of xe2x80x9caxe2x80x9dxc2x125 xc3x85 is at least 65% of the total volume of all the pores, said alumina containing less than 1% by weight of rare earth elements and being loaded with one or more elements of the platinum group.
The third aspect of the invention concerns methods for removing organic compounds comprising bringing the organic compounds into contact with either one of the catalysts of the invention. The invention will hereinbelow be illustrated in more detail.
The first aspect of the present invention will be described.
The catalyst according to the first aspect of the invention comprises a mixture of a zeolite and a metal oxide or oxides containing at least one of the elements of the platinum group.
Zeolites are crystalline aluminosilicates that are represented generally by the chemical composition:
M2/nOxc2x7Al2O3xc2x7ySiO2xc2x7zH2O
where n is the valence of cation M, y is a number of 2 or greater, and z is a number of zero or greater. There are many kinds of known natural and synthetic zeolites. Zeolites suitable for use in the present invention are not restricted to any specific one. In order to attain a high durability or a long service time, however, it is preferred to employ zeolites having an SiO2/Al2O3 molar ratio of 10 or higher in the above-defined chemical composition. As typical examples, there may be mentioned ferrierite, Y, erionite, mordenite, ZSM-5, ZSM-11, beta, etc. These natural and synthetic zeolites may be used as they are or they may be ion-exchanged or calcined before being used in the invention. Preferably, the zeolite to be used in the invention is ion-exchanged. In this case, though the cation species used in ion-exchange is not limited to any specific species, preferably a species from Group IA and/or IIA, and more preferably from Group IIA of the Periodic Table, is used for ion-exchanging purposes. A particularly preferred cation is calcium ion. Two or more cationic species may be used in combination.
The xe2x80x9cmetal oxide(s)xe2x80x9d which may be used in the invention include oxide(s) of metal(s) from Groups IVA, VA, VIA, VIIA, VIII, IB, IIB, IIIB, IVB and VB of the Periodic Table. Amongst them, alumina, titanium oxide, zirconium oxide and silica are preferred.
It is known that alumina is porous. Generally, the size of pores in alumina materials is not uniform and varies widely. Thus, the radii of pores of an alumina material fall in a range of distribution. Where an alumina is employed as a metal oxide material in the first aspect of the present invention, preferably the alumina has such a pore size distribution that, where xe2x80x9caxe2x80x9d represents a pore radius in xc3x85 at the maximum of the pore size distribution curve, the accumulated pore volume of pores having radii in the range of xe2x80x9caxe2x80x9dxc2x125 xc3x85 is at least 65% of the total volume of all the pores. This is because it has been found that an alumina material showing such pore size characteristics enables efficient removal of organic compounds to be achieved easily by strongly promoting combustion thereof. Though the mechanism by which combustion is promoted significantly as a consequence of the stated pore size characteristics has not yet been made clear, it has been found that the performance of a combustion catalyst is largely affected by a pore size xe2x80x9cdistributionxe2x80x9d rather than the pore size itself, i.e., the xe2x80x9cradiusxe2x80x9d.
Further, where an alumina is employed as a metal oxide material in the first aspect of the present invention, it is preferable that the alumina material has a content of rare earth elements of not greater than 1% by weight, preferably not greater than 500 p.p.m. and more preferably it contains rare earth elements in an amount less than a detectable level. The xe2x80x9crare earth elementsxe2x80x9d referred to herein include elements of atomic numbers 57-71 and scandium and yttrium. Usually the content of rare earth elements in alumina may be determined by analyzing the elements using ICP (inductively coupled plasma) emission spectroscopy. A content of not greater than 1% by weight of rare earth elements in the alumina material will enable a highly effective combustion of organic compounds to be achieved easily. Elements other than the rare earth elements may be present in the alumina material without adversely affecting the performance of the catalyst.
The xe2x80x9celement(s) of the platinum groupxe2x80x9d as mentioned herein include six elements that are ruthenium, osmium, rhodium, iridium, palladium and platinum. The most preferred is platinum.
The process for loading the element of the platinum group on the metal oxide is not limited to any specific one. For example, an impregnation technique, ion-exchanging technique or the like may be used for introducing the element of the platinum group into the metal oxide. Where, for example, platinum is to be introduced into a metal oxide material by the ion-exchanging technique, the oxide material is added into a solution containing platinum ions and the resulting mixture is stirred at about 20-100xc2x0 C. for a period of about 5 minutes-100 hours. In the case where an alumina material is to be impregnated with platinum, for example, the alumina material may be added into a solution containing platinum ions and thereafter the solvent, e.g. water, may be removed from the mixture. Examples of the platinum salts which may be used include the ammine complex salt, the dinitrodiammine complex, the chloride salt and the like.
Though the content of the element of the platinum group contained in the metal oxide is not restricted to any specific range, preferably the content ranges from 0.0005 to 10.0% by weight, more preferably from 0.01 to 8.0% by weight, on the basis of the total weight of the metal oxide and the element of the platinum group, in order to obtain an enhanced performance catalyst.
Though the ratio of the metal oxide containing at least one element of the platinum group to the zeolite is not restricted to any specific range, preferably the ratio ranges from 1:20 to 20:1 by weight to obtain a particularly satisfactory catalyst.
The catalyst according to the invention comprises a mixture of a zeolite and a metal oxide containing at least one element of the platinum group. Preferably the mixture is homogeneous, and it is preferable for both the materials to be in powdered form. The mixture may be prepared by any appropriate technique. The materials may be combined together in powder form or slurry form so as to provide eventually a homogeneous mixture. As an alternative method, it is possible to prepare a mixture by mixing a raw metal oxide and a zeolite, and thereafter load at least one element of the platinum group on the metal oxide in the resulting mixture.
The combustion catalysts for removing organic compounds according to the first aspect of the invention may be pretreated, for example, by drying, dehydration, calcination or the like before they are supplied to any intended combustion processes.
The combustion catalysts for removing organic compounds according to the invention may be supplied in any form, such as powder, pellets, honeycomb structure or other appropriate geometric configuration. For example, a binder, such as alumina sol, silica sol or a clay, may be added to the catalyst to be formed into any desired shape or configuration. Alternatively, water may be added to the catalyst powder to prepare a slurry which is applied onto, for example, the surface of a honeycomb refractory substrate made of alumina, magnesia, cordierite or the like.
Now the second aspect of the present invention will be described. The second aspect concerns a combustion catalyst for removing organic compounds, which comprises an alumina having such a pore size distribution that, where xe2x80x9caxe2x80x9d represents a pore radius in xc3x85 at the maximum of the pore size distribution curve, the accumulated pore volume of pores having radii in the range of xe2x80x9caxe2x80x9dxc2x125 xc3x85 is at least 65% of the total volume of all the pores, said alumina containing less than 1% by weight of rare earth elements and being loaded with one or more elements of the platinum group.
It has been found that employment of an alumina material showing such pore size characteristics in the pore size distribution enables efficient removal of organic compounds to be achieved easily by greatly promoting combustion thereof. Though the mechanism by which combustion is promoted significantly as a consequence of the stated pore size characteristics has not yet been made clear, it has been found that the performance of a combustion catalyst is largely affected by a pore size xe2x80x9cdistributionxe2x80x9d rather than the pore size itself, i.e., the xe2x80x9cradiusxe2x80x9d.
It is essential that the alumina contains not greater than 1% by weight, preferably not greater than 500 p.p.m. and more preferably less than the detectable level, of rare earth elements, such as elements of atomic numbers 57-71 and scandium and yttrium. Use of an alumina material having such a content of rare earth elements will enable highly effective combustion of organics to be achieved. Elements other than the rare earth elements may be present in the alumina material without adversely affecting the performance of the catalyst. Incidentally, the content of rare earth elements in alumina may be determined by analyzing the elements using ICP (inductively coupled plasma) emission spectroscopy.
In the second aspect of the invention, the element of the platinum group or elements that is/are to be contained in the alumina material are selected from six elements of ruthenium, osmium, rhodium, iridium, palladium and platinum as mentioned hereinbefore with respect to the first aspect of the invention. Similarly, the most preferred is platinum.
The process for loading the element of the platinum group on the alumina material is not limited to any specific means. For example, as previously mentioned with respect to the first aspect, an impregnation technique, ion-exchanging technique or the like may be used for introducing the element of the platinum group ingredient into the alunima material.
Though the content of the element of the platinum group contained in the alumina material is not restricted to any specific range, preferably the content ranges from 0.0005 to 10.0% by weight, more preferably from 0.01 to 8.0% by weight, on the basis of the total weight of the alumina and the element of the platinum group, in order to obtain an enhanced performance catalyst.
The combustion catalysts for removing organic compounds according to the second aspect of the invention may be pretreated before use as previously mentioned with respect to the first aspect catalysts. Similarly, the second aspect catalysts may be supplied in any suitable shape or configuration or structure.
The third aspect of the invention will be now illustrated. The third aspect concerns a process for removing organic compounds, said process comprising the step of contacting organic compounds with either one of the catalysts according to the first and second aspects of the invention.
The organic compounds which may be removed or destroyed by the invention are carbon compounds which may contain hydrogen, halogen atoms, oxygen etc. in the molecule. As non-limiting examples, there may be mentioned methane, ethane, propane, ethylene, propylene, butadiene, benzene, xylenes, toluenes, chloroform, dichloromethane, trichloromethane, carbon tetrachloride, methylbromide, 1,2-dichloroethane, vinyl chloride, monochlorobenzene, chlorofluorocarbons, PCBs, dioxins, etc. Amongst halogen-containing organic compounds and/or organic compounds showing a vapor pressure of 0.01 kPa or higher at a temperature of 293.15xc2x0 K, hydrocarbons containing two carbon atoms and/or C2 chlorinated hydrocarbons may be effectively treated in accordance with the invention.
The xe2x80x9chalogensxe2x80x9d as used herein include fluorine, chlorine, bromine and iodine. The expression xe2x80x9corganic compounds showing a vapor pressure of 0.01 kPa or higher at a temperature of 293.15xc2x0 K.xe2x80x9d is a measure for defining volatile organic compounds including, for example, methane, ethane, propane, ethylene, propylene, butadiene, benzene, xylenes, toluenes, chloroform, dichloromethane, trichloromethane, carbon tetrachloride, methylbromide, 1,2-dichloroethane, vinyl chloride, monochlorobenzene, chlorofluoro-carbons etc.
When a gas stream containing organic compounds or substances is to be treated for combustion of the organics in the present process, it is preferred that the organics are present initially in a concentration of less than about 1%.
Though the process conditions, such as space velocity and temperature, are not restricted to any specific ranges thereof, a preferred range of the space velocity is from about 100 to 500,000 hrxe2x88x921 and a preferred range of the operation temperature is from about 100 to 700xc2x0 C.
When a gas stream containing organic compounds is subjected to the present combustion process, the raw gas stream may contain other substances, such as water, oxygen, hydrogen, hydrogen chloride, nitrogen oxides, sulfur dioxides, hydrocarbons and fine particulate objects.